Why does the same number work from my mobile but 404 from our office?

Your mobile carrier's routing knows the number; something in your chain (your dial plan translation or your provider's table) doesn't. The number exists — the 404 is local to your path, and the Request-URI comparison will show what your system actually sent.

A number we own returns 404 to callers.

Your inbound side is the patient: the DID isn't built/provisioned on the system the carrier delivers to, or the port into your current provider didn't provision completely. This is a provider ticket with the DID, the port date and an example call — and it outranks most faults because callers blame you, not the routing table.

Can 404 mean the person left and their extension was deleted?

Yes — the commonest internal 404. The system is honestly reporting the extension doesn't exist. Fix the directory/auto-attendant references that still route callers to it.

Troubleshooting · SIP Codes

SIP 404 Not Found: No Such Number Here

By the VoIP Cost Guide Editorial Team | Last updated: October 2026

404 is a routing verdict, not a connectivity verdict: your request reached a working system, and that system states that the destination in the request URI does not exist there. The number may exist in the world — it may even be yours — but on this server, trunk or route, it is unknown. Almost every 404 is a mismatch between the digits you sent and the digits some dial plan expected.

Because 'there' is doing heavy lifting, the first question is always: which element answered 404? Your own PBX, your provider, and the far carrier produce identical codes for very different reasons.

The usual mismatches

Format mismatch: the same number exists in multiple dialects — 10-digit, 1+10, +1 E.164, extension-only. A trunk expecting E.164 404s a 10-digit attempt; a PBX pattern written for 11 digits 404s the 10-digit habit of your users.
Porting in flight: during LNP transitions, routing databases converge at different speeds; the old carrier's switch may 404 a number that ported away hours ago — and the new carrier may 404 it because provisioning lagged the port.
Unassigned or deleted target: the extension/DID was removed, never built, or built on a different PBX/site than the route delivers to — multi-site systems 404 internal numbers that live on the other node when a routing rule is missing.
Wrong route selection: least-cost routing sends the call to a carrier that doesn't serve that destination (a national trunk asked to deliver an international format, or a trunk group missing a rate center).
A number that genuinely doesn't exist: it happens — disconnected numbers and misdials return honest 404s from the far network; the check is calling from an unrelated phone.

Read the ladder before fixing

In the capture, look at the Request-URI of the INVITE and the identity of the 404 sender. The URI shows the digits as translated by everything upstream — frequently not what the user dialed (your own PBX may have stripped a 9 or a country code). The sender tells you whose database lacks the number. A 404 from your own PBX never left your building; a 404 from the provider's SBC means your format met their dial plan and lost; a 404 after several 1xx progress messages means the call traveled and the far side disowned the destination.

Fix sequence

Reproduce with the exact failing number and capture the INVITE: record the Request-URI as sent and the element that returned 404.
Compare formats: dial the same destination in its other dialects (with/without country code, with/without leading 1). A format that succeeds names the translation bug.
Check your dial plan's translation rules for that pattern (prefix stripping/adding, route selection) — fix the rule, not the user's habit.
If the number recently ported (either direction), treat it as convergence: gather the port FOC details and open a ticket with the gaining carrier if the number is yours and still 404s after the port window.
For internal 404s in multi-site systems, verify the destination exists on the node the route delivers to and that inter-node routing tables are synchronized.
Retest all dialects after the fix — dial plans have a habit of fixing one format by breaking another.

Frequently asked questions

Why does the same number work from my mobile but 404 from our office?: Your mobile carrier's routing knows the number; something in your chain (your dial plan translation or your provider's table) doesn't. The number exists — the 404 is local to your path, and the Request-URI comparison will show what your system actually sent.
A number we own returns 404 to callers.: Your inbound side is the patient: the DID isn't built/provisioned on the system the carrier delivers to, or the port into your current provider didn't provision completely. This is a provider ticket with the DID, the port date and an example call — and it outranks most faults because callers blame you, not the routing table.
Can 404 mean the person left and their extension was deleted?: Yes — the commonest internal 404. The system is honestly reporting the extension doesn't exist. Fix the directory/auto-attendant references that still route callers to it.

Sources and further reading

Related guides

Disclaimer: This guide is educational information based on published VoIP standards and professional practice, not engineering advice for your specific system. VoIP pricing, taxes and fees change and vary by provider and location — verify current figures with providers before purchasing. If your phone system supports emergency calling, work with your provider or a qualified VoIP engineer for changes that affect E911.